1. Field of Invention
This invention relates to a motor control circuit and, in particular, to a control circuit suited for the control of direction and magnitude or rotation of a motor on a damper of a heating and ventilating system.
2. Brief Statement of the Prior Art
Heating and ventilating systems commonly employ local zone dampers which are actuated between open and closed positions by a motor that is controlled by means responsive to the temperature sensed by a thermostatic device in the control zone. Commonly, air circulated to the control zone is heated and/or cooled in hot and cold decks of central, roof top air conditioning systems and each control zone in a building has an air duct that receives air from either or both of the heating and cooling decks. The dampers to the hot and cold decks, which proportion the hot and cold air supplied to the air ducts leading to each control zone are usually mounted on a common shaft that is actuated by a single damper motor mechanically connected thereto by levers, screw threads and the like. In some systems, the dampers have a mechanical or electrical feedback so that the control of the motor is proportional to the displacement of the damper, thereby minimizing some of the hunting or oscillation of this drive motor.
To minimize fatigue and wear on the mechanical drive system and to minimize wearing of the motor electrical switch contacts, it is desirable to provide means whereby error signals of minor magnitude do not actuate the damper motor. Accordngly, it is desirable to provide dead band means in the motor controller about the command signal input and to provide the circuit with discriminating capability to eliminate response to input signals of less than a predetermined magnitude.